DonMuncy

Factory weight and balance

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I was reading the other post on weight and balance, and just for fun, decided to check mine. Interestingly, the very first entry (from the factory) seems to have an error. The empty weight is shown to be 1930 at an arm of +43.6, but the moment/1000 is listed at 84.08. Unless my calculator has malfunctioned, the moment should be 84.148, or rounding like they did, 84.15. I recognize this is a miniscule difference, and a little unlikely to cause me to fall out of the sky. But just to play the super accuracy game, do you think they wrote down the arm incorrectly, or did the calculation a little off. What do all the other K model owner's W&B say? Mine is an 82 model, serial number 25-0686. 

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18 minutes ago, DonMuncy said:

I was reading the other post on weight and balance, and just for fun, decided to check mine. Interestingly, the very first entry (from the factory) seems to have an error. The empty weight is shown to be 1930 at an arm of +43.6, but the moment/1000 is listed at 84.08. Unless my calculator has malfunctioned, the moment should be 84.148, or rounding like they did, 84.15. I recognize this is a miniscule difference, and a little unlikely to cause me to fall out of the sky. But just to play the super accuracy game, do you think they wrote down the arm incorrectly, or did the calculation a little off. What do all the other K model owner's W&B say? Mine is an 82 model, serial number 25-0686. 

My first one from the factory has an error too.  They added full fuel weight instead of subtracting it.  They went back and crossed it out and changed the empty weight at some point after.

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5 hours ago, ArtVandelay said:

They might have done these calculations by hand back in those days, or maybe sliderulesemoji38.png

I would think not! 1982? We got our first electronic calculator in 1974. Well before then, grandpa had an "adding machine" at home with a big, long arm that went ca-chunk.

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5 hours ago, ArtVandelay said:

They might have done these calculations by hand back in those days, or maybe sliderulesemoji38.png

In Don's case I would hope they had calculators in 1982

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6 hours ago, DonMuncy said:

The empty weight is shown to be 1930 at an arm of +43.6, but the moment/1000 is listed at 84.08. Unless my calculator has malfunctioned, the moment should be 84.148, or rounding like they did, 84.15. 

If you work it backwards it works out by rounding..84.08 x 1000 = 84080, 84080 \ 1930 = 43.5647 (43.6)...... Measure with a Micrometer and cut with an Axe?

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First, how did you figure that out? Second, any idea why they would do it that way. Do you surmise they weigh the plane and use that to locate the center of gravity. I'm not sure I understand how that works. (But the fact I don't understand it is not surprising)

4 hours ago, RLCarter said:

If you work it backwards it works out by rounding..84.08 x 1000 = 84080, 84080 \ 1930 = 43.5647 (43.6)...... Measure with a Micrometer and cut with an Axe?

 

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22 minutes ago, DonMuncy said:

First, how did you figure that out? Second, any idea why they would do it that way. Do you surmise they weigh the plane and use that to locate the center of gravity. I'm not sure I understand how that works. (But the fact I don't understand it is not surprising)

 

no clue how Mooney did it. As far as me figuring out, I like numbers...;), not very good at them any more since calculators, spreadsheets and my age

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I answered my own question. When I started redoing the calculations in my records, I figured out that is exactly how it is done. The way that RLCarter says. Calculate the Moment of the new item from the weight and arm. Add the new item Moment to the old total Moment (to derive the new total Moment) Then multiply that Moment by 1000 and divide the result by the new total weight to get the new  total arm. Every A&P and avionics tech knows how to do this. Duh. I guess I'm the only one that had not gone through that process before.

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Has someone created an app to calculate W&B changes. I am doing mine by hand and it is driving me crazy.

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Excel spreadsheet. W x A = M

add them up at the bottom 

Edited by jetdriven
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49 minutes ago, jetdriven said:

Excel spreadsheet. W x A = M

add them up at the bottom 

Then divide by new total weight for CG.

New weight = old weight - (removed total weight) + (installed total weight)

Just add the columns up for Removed Items and Installed Items. 

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1 hour ago, DonMuncy said:

Has someone created an app to calculate W&B changes. I am doing mine by hand and it is driving me crazy.

Spreadsheets make it pretty easy.   I have a decent spreadsheet that I adapted from somebody else for a J model, which should be close to a K.  PM me if you want a copy.

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There are apps for that as well...

Foreflight has a WnB section...

I am still using an app call WnB...

Some apps have most of the data and arms already in them...  some apps allow users to share data... so an MSer could email his details to the next MSer... I think I once shared WnB data with Cris around nine years ago...   so long ago, it felt like I was sharing my fingerprints or something... :)

Great news for Don... doing your own weight and balance calculations puts you half way to having an engineering degree... see the class called statics... great for bridge building and things like that... dynamics is the next level cooler... accelerations in 3D...  :) (iirc Don has a collection of degrees)

Best regards,

-a-

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11 minutes ago, carusoam said:

There are apps for that as well...

Foreflight has a WnB section...

I am still using an app call WnB...

Some apps have most of the data and arms already in them...  some apps allow users to share data... so an MSer could email his details to the next MSer... I think I once shared WnB data with Cris around nine years ago...   so long ago, it felt like I was sharing my fingerprints or something... :)

Great news for Don... doing your own weight and balance calculations puts you half way to having an engineering degree... see the class called statics... great for bridge building and things like that... dynamics is the next level cooler... accelerations in 3D...  :) (iirc Don has a collection of degrees)

Best regards,

-a-

Those all require Gross Weight and empty CG, and that's what the OP needs to recalculate. 

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Yes, Anthony, I have had a bunch of education, but never took Statics.

I managed to wade through all my W&B calculations, for a net useful load gain of 2 pounds due to errors, and less than 1/2 inch in CG.  Actually, except for rounding errors, the only change was where my avionics shop managed to overlook one sheet back in '04, which then tainted all the numbers up until now.

So, mainly an exercise in futility, except for the satisfaction of knowing it is right, and I now know how to do the calculations. 

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1 minute ago, DonMuncy said:

Yes, Anthony, I have had a bunch of education, but never took Statics.

. . .

So, mainly an exercise in futility, except for the satisfaction of knowing it is right, and I now know how to do the calculations. 

Don't feel bad, Don. I had two required EE courses (Circuit Analysis I & II), but I can't do those calculations now (only V = IR stuck). So you've got a leg up on this overeducated ME.

          --Hank, BSME, MSE, PPL, IA and some more without cool abbreviations . . . . .

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I wanted good numbers because of errors that had be recorded over the years so we re-weighed and did the math...3lb gain in UL and GC was less than 0.250" 

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Poof that re-weighing the plane can be good... and re-doing the math can be beneficial as well!

:)

Best regards,

-a-

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I would add that recalculating all your W&Bs is not an exercise for the weak at heart. The weights are easy. The moments and CGs are entirely different. The worse part is that they are not intuitive, at least for me. Removing an item generally results in a minus to the moment, but if you change a prop, you will wind up doing a minus for the removal, but it is at a negative station, so the two minuses turn into a positive. How does anyone who is not a mathematical genius wrap their head around that.;) Living with some minor errors starts looking pretty nice.

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12 hours ago, DonMuncy said:

I would add that recalculating all your W&Bs is not an exercise for the weak at heart. The weights are easy. The moments and CGs are entirely different. The worse part is that they are not intuitive, at least for me. Removing an item generally results in a minus to the moment, but if you change a prop, you will wind up doing a minus for the removal, but it is at a negative station, so the two minuses turn into a positive. How does anyone who is not a mathematical genius wrap their head around that.;) Living with some minor errors starts looking pretty nice.

If you understand torque, the moments are just the torque that that item puts around the datum, wherever that is, due to gravity.   If the aircraft were supported only at the datum and all the moments added to zero, it would balance on the datum.   This is why the arms are all taken from the datum.   It's also why the sign changes in front of the datum, because the torque goes the opposite direction (counterclockwise viewed from the left wingtip for stuff in front of the datum, clockwise for stuff behind it).

When you're all done and add up all the torques (moments, with their appropriate signs), there will be a net torque (aka moment) left over.   That torque at the datum is from the weight of the aircraft at its center of gravity.   So when you're all done adding and subtracting moments, if you divide out the weight of the aircraft from the final moment quantity, you will have the arm about the datum to the center of gravity.

It is actually very basic stuff, but seldom gets explained or demonstrated to people in a way that they might understand easily.   It is also why sometimes the datum is out in front of the airplane, so that there are no sign changes and errors cannot happen due to a sign mistake.   That does, however, make it more difficult to measure an arm for addition of a new item, so there's a tradeoff between making the datum easily locatable and increasing the likelihood of sign errors in calculations. 

Edited by EricJ

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Every time I weigh an airplane, I find errors. CG shifts of two inches are common. 40 to 60 pound errors in the empty weights are common when the plane has been weighed with full fuel and then calculated out. 

Factory weights are usually incorrect, because the factory may weigh a plane that is produced for inventory, and it is not fully equipped yet. Then a dealer has the factory add the preferred equipment, or the dealer adds it. So the original factory weight and balance may have the notation: "W&B as delivered on 1-23-1978", when it was "delivered" to factory inventory, not to the end customer. It may have not had any radios at all, no autopilot, etc. And then the additional equipment W&B corrections get missed, or lost. Happens more often than you might think.

Just weigh it. And expect it to be heavier than you think.

Edited by philiplane

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1 hour ago, EricJ said:

If you understand torque, the moments are just the torque that that item puts around the datum, wherever that is, due to gravity.   If the aircraft were supported only at the datum and all the moments added to zero, it would balance on the datum.   This is why the arms are all taken from the datum.   It's also why the sign changes in front of the datum, because the torque goes the opposite direction (counterclockwise viewed from the left wingtip for stuff in front of the datum, clockwise for stuff behind it).

When you're all done and add up all the torques (moments, with their appropriate signs), there will be a net torque (aka moment) left over.   That torque at the datum is from the weight of the aircraft at its center of gravity.   So when you're all done adding and subtracting moments, if you divide out the weight of the aircraft from the final moment quantity, you will have the arm about the datum to the center of gravity.

It is actually very basic stuff, but seldom gets explained or demonstrated to people in a way that they might understand easily.   It is also why sometimes the datum is out in front of the airplane, so that there are no sign changes and errors cannot happen due to a sign mistake.   That does, however, make it more difficult to measure an arm for addition of a new item, so there's a tradeoff between making the datum easily locatable and increasing the likelihood of sign errors in calculations. 

Like I said, "unless you are a mathematical genius...." :D 

Yes, in a sense I understand, but it is still difficult. And yes, it would be a little easier to calculate if the datum was in front of the plane, rather than somewhere near the panel like Mooney does it.

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